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The Purpose of the High Voltage Transformer Used in Microwave Ovens

Excerpts from the book The Complete Microwave Oven Service Handbook --Available on CD-ROM (CLICK HERE)
and from the video You Can Fix Your Microwave Oven - Copyright ©, 1989-2008 by J. Carlton Gallawa

© 1996-2008 by J. Carlton Gallawa . All Rights Reserved Worldwide

It might be said that the HIGH-VOLTAGE TRANSFORMER is the "muscle" of the microwave oven. With an input of 120 VAC (or 240 VAC in many commercial models) applied to the primary winding, the high-voltage transformer (also referred to as power or plate transformer) steps up that primary voltage to a very high voltage. This high voltage is then boosted even higher by the voltage-doubling action of the capacitor and diode. The resulting voltage, about 3000 - 5000 volts DC (depending on the model), is available at the high voltage (output) tap (see illustration below).

In most models, the transformer also incorporates a second output winding, as shown in the illustration below. This is a stepped-down output that provides the filament voltage (typically 3 to 4 VAC) to the magnetron tube. DO NOT attempt to measure this voltage.

Some older models use a separate filament transformer . If that is the case, skip steps 3 through 5.

Important Safety Information

Microwave ovens are one of the most dangerous appliances to work on. Before attempting any troubleshooting, testing or repairs, for your personal safety we strongly urge you to carefully read, fully understand and be prepared to follow carefully the very important safety precautions found by clicking here .

Please read our disclaimer at the bottom of this page.

WARNING! THESE VOLTAGES CAN BE LETHAL! NEVER, EVER ATTEMPT TO MEASURE THE OUTPUT VOLTAGES OF THE HIGH-VOLTAGE TRANSFORMER.!

typical high voltage transformer

How To Test The High-Voltage Transformer

  1. UNPLUG the oven.
  2. DISCHARGE ALL HIGH-VOLTAGE CAPACITORS. (Procedure)
  3. Disconnect the high-voltage lead from the transformer's high-voltage terminal (or tap).
  4. With an ohmmeter set to the lowest resistance scale, R X 1, measure the resistance from the high-voltage terminal to the transformer chassis (ground).
    1. Some early models are equipped with two or three high-voltage taps. If this is the case, perform the measurement from each tap to chassis ground.
  5. The meter should read about 55 to 70 ohms, depending on which high-voltage tap is being measured and the model being tested.
    1. A substantially higher or lower reading would indicate that the transformer is defective.
  6. Carefully disconnect the leads from the primary (input) terminals
  7. Measure from one transformer primary terminal to the other for a normal reading of less than 1 ohm (i.e., .22 ohms)
    1. A substantially higher reading would indicate that the primary winding is open.
  8. Set the meter to its highest resistance scale and check from each transformer primary terminal to the transformer chassis (ground) for a normal reading of infinity (open circuit).
    1. Any measurement of resistance would indicate some degree of a short to ground
  9. Set the meter back to its lowest resistance scale. Then carefully disconnect the filament leads and measure from one filament terminal to the other for a normal resistance of less than 1 ohm.
  10. Set the meter to its highest scale and measure from each filament terminal to chassis ground for a normal reading of infinity (open circuit).
    1. Any measurement that is substantially different from the normal readings would indicate a defective transformer.



Illustration from THE COMPLETE MICROWAVE OVEN SERVICE HANDOOK
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Operation, Maintenance, Troubleshooting and Repair
J. Carlton Gallawa

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Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted © 1989-2008 by J. Carlton Gallawa. The material available through this site may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

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